In recent years, the use of cellular communication systems having mobile terminals which communicate with a hardwired network, such as a local area network (LAN) and a wide area network (WAN), has become widespread. Retail stores and warehouses, for example, may use cellular communications systems to track inventory and replenish stock. The transportation industry may use such systems at large outdoor storage facilities to keep an accurate account of incoming and outgoing shipments. In manufacturing facilities, such systems are useful for tracking parts, completed products and defects.
A typical cellular communication system includes a number of fixed base stations interconnected by a cable medium to form a hardwired network. The hardwired network is often referred to as a system backbone. Also included in many cellular communication systems are intermediate base stations which are not directly connected to the hardwired network. Intermediate base stations, often referred to as wireless base stations, increase the area within which base stations connected to the hardwired network can communicate with mobile terminals. Unless otherwise indicated, the term "base station" will hereinafter refer to both base stations hardwired to the network and wireless base stations.
Wireless communication systems such as the cellular communication system described above often involve spread spectrum (SS) technology. An SS communication system is one in which the transmitted frequency spectrum or bandwidth is much wider than absolutely necessary. Generally, SS technology is used by those who wish to communicate in the unlicensed bands provided by the FCC. These bands include the 902 through 928 MHZ and 2.4 through 2.48 GHz ranges. The FCC requires that information transmitted in the bands be spread and coded in order to allow multiple users to communicate in these bands at the same time.
One type of a SS communication system is known as a direct sequence spread spectrum (DSSS) system. The coding scheme for a DSSS communication system utilizes a pseudo-random binary sequence (PRSB). In a DSSS system, coding is achieved by converting each original data bit (zero or one) to a predetermined repetitive pseudo noise (PN) sequence or code. A type of PN sequence is illustrated in FIG. 1. For this example, the digital data signal 110 is made up of a binary "1" bit and a "0" bit. A PN sequence 120 representing the digital data signal 110 is comprised of a sequence of ten sub bits or chips, namely, "1", "0", "1", "1", "0", "1", "1", "1", "0", "1".
The digital data signal 110 is coded or spread by modulo 2 multiplying (e.g., via an "EXCLUSIVE NOR" (XNOR) function) of the digital data signal 110 with the PN sequence 120. If the data bit is a "1", then the resulting spread data signal (PN coded signal) in digital form corresponds to the PN sequence 120. However, if the data bit to be coded is a "0", then the spread data signal in digital form will correspond to a code 130. As can be seen, the code 130 is the inverse of PN sequence 120. That is, the PN sequence and its inverse are used to represent data bits "1" and "0" respectively.
FIG. 2 schematically illustrates a transmitter portion or assembly 100 of a DSSS system. Original data bits 101 are input to the transmitter portion 100. The transmitter portion includes a modulator 102, a spreading function 104 and a transmit filter 106. The modulator 102 modulates the data onto a carrier using, for example, a binary phase shift keying (BPSK) modulation technique. The BPSK modulation technique involves transmitting the carrier in-phase with the oscillations of an oscillator or 180 degrees out-of-phase with the oscillator depending on whether the transmitted bit is a "0" or a "1". The spreading function 104 converts the modulated original data bits 101 into a PN coded chip sequence, also referred to as spread data. The PN coded chip sequence is transmitted via an antenna so as to represent a transmitted PN coded sequence as shown at 108.
FIG. 2 also illustrates a receiver portion or assembly, shown generally at 150. The receiver portion 150 includes a receive filter 152, a despreading function 154, a bandpass filter 156 and a demodulator 158. The PN coded data 108 is received via an antenna and is filtered by the filter 152. Thereafter, the PN coded data is decoded by a PN sequence despreading function 154. The decoded data is then filtered and demodulated by the filter 156 and the demodulator 158 respectively to reconstitute the original data bits 101. To receive the transmitted spread data, the receiver portion 150 must be tuned to the same predetermined carrier frequency and be set to demodulate a BPSK signal using the same predetermined PN sequence.
In order to receive an SS transmission signal, the receiver portion must be tuned to the same frequency as the transmitter assembly to receive the data. Furthermore, the receiver portion must use a demodulation technique which corresponds to the particular modulation technique used by the transmitter assembly (i.e. same PN sequence length, same chipping rate, BPSK). Because mobile terminals communicate with a common base station, each device in the cellular network must use the same carrier frequency and modulation technique.
A PN sequence length refers to a length of the coded sequence (the number of chips) for each original data bit. As noted above, the PN sequence length effects the processing gain. A longer PN sequence yields a higher processing gain which results in an increased communication range. The PN sequence chipping rate refers to the rate at which the chips are transmitted by a transmitter portion. A receiver portion must receive, demodulate and despread the PN sequenced chip sequence at the chipping rate utilized by the transmitter portion. At a higher chipping rate, the receiver portion is allotted a smaller amount of time to receive, demodulate and despread the chip sequence. As the chipping rate increases so to will the error rate. Thus, a higher chipping rate effectively reduces communication range. Conversely, decreasing the chipping rate increases communication range.
The spreading of a digital data signal by the PN sequence does not effect overall signal strength (or power) the data being transmitted or received. However, by spreading a signal, the amplitude at any one point typically will be less then the original (non-spread) signal.
Data throughput rate has become an ever increasingly important factor in the communication market. Market forces have been dictating the need for the ability to communicate in real-time and to be able to exchange data with high speed, low error and efficiency. In order to increase data throughput rate, conventional DS systems and/or hybrid systems using conventional DS technology typically have accepted a loss in transmission range or used larger amounts of bandwidth to transmit the data. However, transmission range and spectral bandwidth efficiency are also important elements to DS systems. Therefore, there is a strong need in the art for a method and system for improving data throughput rate without necessarily sacrificing data transmission range, spectral efficiency or other critical elements of a DS system.